In electrical engineering, in the field of pulsed power technology, high-voltage and high power pulses of amplitude ranging from a few kW to several hundred TW are used for scientific and industrial purposes, whereby pulse durations lie within the ps to ms range.
For “electroporation”, which is specified here as an example of an industrial application, a pulse generator is required which is capable of generating, for example, voltages of 250 kV and currents of several 10 kA with a pulse duration ranging from 1 μs to 2 μs.
A potential topology for the execution of a pulse generator of this type is an “inductive voltage adder”, abbreviated to “IVA”. A generator of this type can employ a compact form of construction on the grounds that, during pulse generation, the latter includes a series circuit of n discrete voltage sources.
In the stationary state, the voltage sources are wired together in parallel, during the pulse phase, according to the related art, the latter are uncoupled by an inductance and, as dictated by the topology of the IVA, are combined to form a series circuit.
“Pulsed Power Systems Principles and Applications”, Hansjoachim Bluhm, Springer Verlag Berlin Heidelberg, 2006, specifically in chapter 7, pages 192-201, discloses the structure and operating methods of IVAs.
The value of the inductance for this purpose is defined by the geometry and the susceptibility of the core material. The inductance must show a sufficiently high response to the pulse to permit the mutual formation of a series circuit of voltage sources, while preventing any substantial losses associated with the flow of currents via the inductances on the ground side.
Accordingly, for pulse durations e.g. of 0.1 to 50 μs, the inductance for the series connection of the voltage sources essentially dictates the volume and the costs of the IVA.
In an IVA, the wave properties associated with reflection factors can be employed, during the closing operation and in the stationary state, to increase the current amplitude. FIG. 1 shows a known circuit layout of an IVA. FIG. 1 shows the basic layout of an IVA, based upon the example of four stages. In a similar manner to a series arrangement of voltage sources, represented on the left-hand side of FIG. 1, pulse cables, as represented on the right-hand side of FIG. 1, may be configured as voltage multiplier circuits, wherein the positive conductor of one cable is connected to the negative conductor of another. Given that, in this alternating connection of conductors, no short-circuit is produced, the connection must be isolated for the duration of the pulse. As is known, this is achieved by the employment of transmission lines of sufficient length, in the form of a cable transformer, or by coupling with sufficiently high coupling inductances, corresponding to the form of embodiment of an IVA. A more compact form of construction is possible if the transfer time is replaced by an inductive infeed associated with the combination of the pulse cables of individual stages. The principle of voltage addition by magnetic infeed, in accordance with the IVA, is represented in FIG. 2.
FIG. 2 shows a known example of an embodiment of an IVA with magnetic isolation. FIG. 2 shows six stages, which are arranged coaxially. Reference number 1 designates a vacuum interface, reference number 3 designates a vacuum, reference number 5 designates an annular gap, reference number 7 designates a magnetic core, reference number 9 designates a particle stream gap for the generation of electron or ion streams in a vacuum, and reference number 11 designates oil. The cylindrical cavities form an internal conductor of the IVA, and are supplied radially by known, coaxially-arranged voltage sources Ux. In the applications described, each of the individual cavities delivers a pulse, for example of duration 0.1 to 50 μs, with a voltage amplitude U0 of several kV, for example in the range of 3 to 10 kV, and a maximum current amplitude I0 ranging from several kA to >10 kA. In order to achieve the addition of the voltage amplitudes, the vector addition of electromagnetic fields in the transition zone to the coaxial transmission line is exploited. Accordingly, the IVA generates a voltage pulse by the superimposition of the sum of the n individual voltage sources (where n is the number of stages). Correspondingly, an arrangement as shown in FIG. 2 generates a six-fold voltage pulse, in relation to the voltage sources Ux. In order to achieve the addition of the voltage amplitudes, the positive conductor of one voltage source is connected to the negative conductor of the next. By definition, a conductive connection between the middle electrode and current-distributing outer electrode in each cavity is formed accordingly. In order to prevent the consequent formation of a short-circuit at the cable output, the impedance of the connection is substantially raised by increasing the relative permeability of the section concerned. To this end, a partial volume of the voltage source is filled with annular strip-wound cores of a ferromagnetic material.
In the literature, this technique is also described as a magnetic switch. The properties of the magnetic switch are dependent upon the ferromagnetic core and the pulse duration. The following formulae apply:
            u      ⁡              (        t        )              =                            d          ⁢                                          ⁢                      ϕ            ⁡                          (              t              )                                                d          ⁢                                          ⁢          t                    =              A        ⁢                              d            ⁢                                                  ⁢                          B              ⁡                              (                t                )                                                          d            ⁢                                                  ⁢            t                                          ∫                        u          ⁡                      (            t            )                          ⁢        d        ⁢                                  ⁢        t              =                            U          0                ⁢        T            =              A        ⁡                  (                                    B              r                        +                          B              r                                )                    
The time interval during which the magnetic core may be considered as unsaturated, specifically a Vs (volts*seconds) product, is given by the cross-section of the annular core and the sum of the remnant and saturation inductance. An appropriate ferromagnetic material must show a high saturation inductance and a steep hysteresis characteristic. Given that, in the known forms of embodiment, the conductor geometry only encompasses a single winding, the cross-sectional area thereof. A must be sufficiently large such that, by this single winding, a sufficiently high inductance can be generated, which will show the required impedance in the frequency range considered. As a further condition, it is required that the magnetic core should not achieve saturation, as the inductance would otherwise fall abruptly to a low value. On these grounds, it proceeds that large volumes are to be filled with a ferromagnetic material, thereby resulting in substantial material costs, large structural volumes and heavy-duty structural arrangements for the management of the large masses involved.